When an airplane is flown during certain atmospheric conditions, ice can accumulate on its exterior surfaces. The formation of this ice can be precipitated either by impingement of atmospheric supercooled water droplets or by leakage of water from internal systems. The formation of ice gives rise to four areas of concern. Firstly, exterior surface ice can break loose and cause damage to engines, protruding surfaces such as antennas, wings and moveable control surfaces, and structures on the ground, as well as result in injury to people on the ground. Secondly, ice accumulations on airfoil sections such as wings and stabilizers can seriously degrade the airfoil's aerodynamic performance by adversely affecting both its lift and drag characteristics.
Thirdly, the weight of ice changes the overall weight of the airplane and also its center of gravity. Finally, water can run back, or downstream, from the leading edge where it impinges or is formed when ice is melted by deicing means, and subsequently freeze on moveable surfaces and interfere with their operation. Consequently, the airplane industry has focused a great deal of attention on the development of devices to either prevent the formation of ice on airplane surfaces or to remove the ice from such surfaces.
Ice protection apparatus are generally categorized as either deicing or anti-icing. Deicing devices allow ice to form and periodically remove it. Anti-icing apparatus prevent the formation of ice on the airplane surfaces. The three general approaches to deicing are thermal, chemical and mechanical.
Thermal deicing raises the temperature of the airfoil surface by heating. One thermal deicing apparatus is a heating element either contained in an elastomeric boot located on a leading edge to be deiced or integrally incorporated into the leading edge. The heating element is powered by electrical energy from a generating source usually driven by the aircraft engine. The leading edge is heated by the heating element until some of the ice melts and the remaining ice is dislodged and swept away by the airstream.
Another thermal deicing approach involves the circulation of gases at elevated temperatures through passageways in the leading edge of an airfoil. The hot gases are supplied by the compressor stages of a turbine engine, resulting in reduced fuel economy and lower turbine power output. Another drawback inherent to all thermal deicing devices is the formation of water from melted ice, called runback water, which runs back and refreezes to form runback ice on downstream surfaces. Runback ice may cause unwanted and unpredictable aerodynamic forces, added weight, interference with the operation of movable surfaces, and damage if a mass of runback ice breaks loose.
Chemical deicing, also known as freezing point depression, is achieved by application of a chemical to the ice through pores in the leading edge. The freezing point of the ice is lowered and it turns to slush, which is swept away by the airstream. The application of the chemical must be repeated as ice reforms and thus the store of chemical must be replenished after each icing flight. Further adding to the maintenance burden is the ongoing requirement to check the application apparatus to ensure its proper operation. Another drawback is the formation of runback water from melted ice, which can form runback ice downstream of the leading edge.
Mechanical deicing devices deform the exterior surface of the airplane to break up the ice and allow it to be swept away by the airstream. Pneumatic boots are the original and most popular device in this class. The boots are expandable tube-like structures typically used to cover the leading edge of an airfoil. The structures are inflated with pressurized fluids and cause the ice to crack. However, the distortion occasioned in an airstream profile by inflation of the tube-like structures can substantially alter the airstream pattern over the airfoil and adversely affect the airfoil's lift characteristics. All of the various mechanical deicing devices remove the ice without melting it, thereby avoiding the creation of runback water and, concomitantly, runback ice.
Two anti-icing approaches are used to protect airplanes from the formation of ice on their external surfaces: evaporative and running wet. Evaporative anti-icing systems completely evaporate all impinging water by heating the leading edge or other area of water impingement to a high temperature. This solution eliminates the problem of runback water and ice, but requires using excessive thermal energy and is costly. Running wet systems melt all of the ice forming in the area where the water is impinging by the use of freezing point depressants or thermal energy provided by heating means, but do not evaporate the resulting water, instead allowing it to run downstream. Although less costly to operate than evaporative anti-icing systems, running wet systems have an inherent drawback in that the runback water flows back over unheated or untreated surfaces of the airfoil to possible form runback ice.
Several techniques are used to avoid the formation of runback water and ice attendant to the use of running wet anti-icing systems. Heating the entire surface from the leading edge to the trailing edge to keep the surface temperature above the freezing temperature will prevent refreezing of the runback water, but requires a very high expenditure of thermal energy and also increases the mechanical complexity of the underlying structure. Using excessive freezing point depressant in the area of impingement tends to prevent refreezing of runback water, but the chemicals inevitably wear off and evaporate, and thus the foregoing solution cannot ensure that runback water will not subsequently begin freezing without intermittent reapplications of the depressant. The process thus requires constant monitoring by the crew of the airplane, and is always limited by the finite reservoir of depressant that can be carried.